The invention relates to an image projection apparatus comprising an illumination system, an image display system having at least one display panel for generating an image to be projected, in which the direction of polarization of an illumination beam supplied by the illumination system is modulated with the image information, a projection lens system for projecting the image formed by the image display system on a projection screen and a polarization-sensitive beam splitter arranged between the illumination system and the image display system for splitting the illumination beam into two differently polarized sub-beams the first of which is directly suitable to be modulated by the image display system and a polarization-rotating element for converting the second sub-beam into a beam having the same state of polarization as the first sub-beam, said first and second sub-beams being incident on the image display system.
The term image projection apparatus should be understood in a wide sense and comprises a device for displaying, for example, a video image, a graphic image, numerical information or a combination thereof. The images may be both monochrome and colour images. In the latter case the display system may comprise three colour channels for, for example, the primary colours red, green and blue, each channel comprising a display panel.
Such an image projection apparatus for a colour image is described in German Patent Application No. 3,829,598. The display panels of the known apparatus are reflective panels having a layer of liquid crystalline material of the so-called nematic type as an active, or image-forming element. This layer changes the direction of polarization of an incident beam of light locally, in dependence upon the image information. To this end the light beam must be polarized linearly in a given direction. The known apparatus comprises a polarization-sensitive beam splitter consisting of two prism portions between which a dielectric interference layer is provided. This layer splits the illumination beam into two mutually perpendicular linearly polarized sub-beams a first of which is passed on to the image display system. To make efficient use of the available light, a .lambda./4 plate, in which .lambda. is the wavelength of the light, is arranged in a diagonal position in the path of the second sub-beam reflected by the interference layer. A reflector reflecting the second sub-beam towards the interference layer is present behind this .lambda./4 plate. Since the second sub-beam has traversed the .lambda./4 plate twice, its direction of polarization is rotated through 90.degree. with respect to the original direction so that the second sub-beam is also passed on to the image display system. Two sub-beams then exit from the polarization-sensitive beam splitter, which sub-beams are located on both sides of a plane through the principal axis of the apparatus. The principal axis of this apparatus is the line which connects the centre of the radiation source in the illumination system to the centre of the image display system.
Since the operation of the interference filter used as a polarization beam splitter is dependent on the wavelength of the radiation and on the angle at which the rays are incident on this filter, this filter is less suitable because the illumination beam has a larger aperture angle and a wavelength range which comprises the entire visible light spectrum. Moreover, the optical path lengths through the beam splitter are different for the two sub-beams so that the two images of the radiation source formed by these sub-beams are located at different positions along the principal axis. The known apparatus does not have any provisions to achieve that the cross-section of the illumination beam at the location of a display panel corresponds to that of the panel and at the location of the projection lens system corresponds to the aperture of this lens system.